In acoustic wave communication for data communication using a speaker and a microphone, acoustic wave output from the transmitter speaker may sometimes reflect on surrounding objects, and the reflected wave may be input to the receiver microphone as a delay wave.
When the delay wave is input to the microphone, the receiver is no longer able to successfully restore the acoustic wave transmitted from the transmitter (that is, transmission data) from the received signal.
Therefore, in acoustic wave communication, the transmitter speaker and the receiver microphone are brought into proximity so that the reflected wave is not to be input to the receiver microphone.
However, it is troublesome to arrange the transmitter and the receiver to be close to each other at the time of establishing acoustic wave communication. Also, there are cases in which close arrangement is not physically possible.
Therefore, in acoustic wave communication, it has been desired to increase a communicable distance between the transmitter and the receiver.
On the other hand, in order to acquire desired communication quality even with a longer communication distance, it has been proposed that the receiver removes unnecessary signal components, such as the reflected wave and the like, from the received signal (see Patent Document 1, for example).
According to the proposed technique, upon converting transmission data to an acoustic wave to be transmitted at the transmitter side, a pseudo-noise code is added to the transmission data, and the receiver, using an adaptive filter compatible with the pseudo-noise code, separates the received signal into a data area and a pseudo-noise area.
Then, the receiver acquires a multipath delay characteristic value of the signal in the pseudo-noise area (pseudo noise code) that is separated from the received signal, and, based on the multipath delay characteristic value, removes unnecessary signal components from the signal in the data area.